Although vortex tubes are well known in the art, such devices have not gained wide acceptance due to a limited understanding of the thermodynamic principles involved. As a result few practitioners have studied the features of gas behavior in a vortex tube or adapted use of the vortex tube into gas separation technology.
First observed in the 1930's, the vortex tube is responsible for the so-called Ranque effect wherein a gas at higher pressure which is throttled in a centrifugal field set up in a tube will separate into two outlet streams: one which is cooler and one which is hotter than the temperature of the gas feed. In a vortex tube, the gas stream is fed tangentially to the tube wall and expanded in the tube. The vortex thus formed creates an intense centrifugal field within which gas dynamic transport processes and to a lesser extent Joule-Thomson (JT) cooling establish temperature, pressure and compositional gradients in the tube both axially and radially. The net result is that the vortex core which becomes cooled flows in the opposite direction to the vortex periphery which becomes heated. The coolest gas occurs at the end of the tube in the cool flow direction and the hottest gas occurs at the end of the tube in the hot flow direction. Vapor components of the feed gas, if close to their dew point, initially condense in the core and are flung to the periphery by centrifugal action. However, condensate thus formed becomes heated and re-vaporized. The fraction of the peripheral stream which does not exit the hot end migrates back to the core and gets re-condensed as it flows in the cool direction. This condensate is then generally flung back to the peripheral stream before it can exit the cold end. As a result, condensate vapors entering the vortex tube with the feed are concentrated in and mostly discharged with the hot stream, and the cold stream exhausts as an essentially dry, saturated stream.
Fulton U.S. Pat. Nos. 3,173,273 and 3,208,229, the disclosures of which are hereby incorporated herein by reference, describes basic designs for most efficient vortex tube operation. The characteristic performance curve for a typical vortex tube as described by Fulton having the hot side insulated and operating under ideal gas conditions, where the Joule-Thomson cooling effect is negligible, is shown in FIG. 1, which is a graph of the hot side and cold side outlet stream temperature change (.DELTA.T.sub.h -.DELTA.T.sub.c) with respect to the feed temperature (T.sub.f) versus the fraction of the feed stream (.chi..sub.c) which exits the tube through the cold end. Typically, the maximum temperature drop in the cold stream is about 50 percent of an adiabatic temperature drop occurring for the same pressure drop at the cold outlet. This generally occurs at a cold fraction of 0.4 or less. In terms of temperature differential alone between the hot and cold streams (.DELTA.T.sub.h -.DELTA.T.sub.c), however, this differential is about 83 percent of the adiabatic temperature drop. As a greater fraction is withdrawn from the cold end up to about 75 percent optimally, the temperature drop in the cold stream alone becomes lower (about 30% at .chi..sub.c =0.75) but the temperature spread (.DELTA.T.sub.h -.DELTA.T.sub.c) increases to about 120 percent of the adiabatic temperature drop for the corresponding pressure drop at the cold outlet. Under real gas conditions, for example at high pressures or low temperatures, the overall cooling experienced in a vortex tube is even larger because the Joule-Thomson cooling effect is superimposed over the Ranque effect.
Advantageous features of a vortex tube are an absence of moving parts and reduced utility requirements in comparison with an expansion turbine, for example.
The cooling effect of a vortex tube has been used in the past to recover liquids from gas streams. Fekete U.S. Pat. No. 3,775,988 describes the use of cold flow vortex tube expansion principles in liquefaction and cold-producing processes wherein the vortex tube substitutes for an expansion turbine. Fekete U.S. Pat. No. 4,458,494 describes the use of an improved vortex tube in a gas-liquid separation process wherein vaporization of the liquid in the peripheral stream of the tube is retarded by either cooling a short section of the periphery with a tube cooling jacket or by taking the liquid out at a short distance from the tube inlet, where the heating effect is minimal, and insulating the liquid from the heating effect.
Shirokov et al. U.S. Pat. No. 4,185,977 describes a process of separating hydrocarbon from gaseous mixtures to produce hydrogen. The process resides in cooling a gas mixture comprising hydrogen, methane and olefins in heat exchangers by stages with the resulting liquid condensate being separated at each stage. The products obtained are a liquid condensate of olefins, liquid methane/hydrogen, and an enriched hydrogen vapor stream. The liquid methane condensate with an admixture of hydrogen is evaporated and expanded with the use of a vortex effect. The resulting cold and hot streams are fed separately to the heat exchangers as a heating or cooling medium. The hydrogen-rich vapor portion is also expanded with the use of a vortex effect to give a cold stream consisting of pure hydrogen whereupon the cold and hot streams are fed separately to heat exchangers each having an appropriate temperature.
Shirokov et al. U.S. Pat. No. 4,257,794 describes a process of separating a gaseous hydrocarbon mixture of methane, olefins and hydrogen residing in cooling the mixture by stages with the resulting liquid condensate of olefins and methane being withdrawn at each stage, recovering a gaseous hydrogen-methane mixture with some mixed ethylene produced by demethanization in a demethanizer column and further cooling the same. The resulting condensate thus withdrawn is adapted for use as a spraying means in the demethanization. The remaining gaseous methane-hydrogen mixture is expanded with application of a vortex effect to produce hot and cold streams. The hot and cold streams are mixed, with the cold stream having first passed countercurrently against the methane-hydrogen-ethylene mixture, and again expanded with application of a vortex effect. The resulting hot stream is fed countercurrently against the hydrocarbon stream being separated and the cold stream countercurrently against the methane-hydrogen-ethylene mixture.
Kosenkov et al. Russian patent publication SU1160211-A describes a hydrocarbon cooling method including the throttling of a liquid stream from a vortex pipe which stream is used for cooling the vortex pipe hot end.
Heat exchanger equipment having a high thermal effectiveness coefficient are described in W. M. Kays and A. L. London, Compact Heat Exchangers, 3rd ed. 1984, New York:McGraw Hill; and D. Chisolm (Ed), Developments in Heat Exchanger Technology-1, Applied Science Publications, 1980, Chapter 5, Usher et al., "Compact Heat Exchangers" which are hereby incorporated herein by reference.
Other references of interest include Atkinson U.S. Pat. No. 2,683,972 and Fekete, The Oil and Gas Journal, Jun. 15, 1970, pp. 71-73.
The cooling effect of the vortex tube has generally been utilized in combination with heat exchangers of low thermal efficiency and has been confined to relatively high pressure applications. It is well known that a minimum pressure ratio for the expansion is necessary to achieve the effect desired in a vortex tube and that increasing the expansion pressure drop enhances the cooling obtained. There are however, many low pressure gas streams containing condensable vapors such as flared waste gas streams and exhaust fumes from sulfide ore smelters where the JT cooling effect is negligible, economic value of the gas stream is low in relation to the cost of compressor equipment and the use of expanders is impractical. It would be advantageous to be able to recover valuable liquids, (e.g., hydrocarbons) from these streams as well.